1. Field of the Invention
The invention relates in general to a method and apparatus for sizing a galvanized tube. Specifically, two sizing stands are used. A first sizing stand with sizing rolls is used prior to the galvanizing process and a second sizing stand with resilient sizing rolls is used after the galvanizing process.
2. Description of the Related Art
There is a continuous need for galvanized tubes which can be shaped and sized yet still retain a durable surface. The U.S. patent to Creps, U.S. Pat. No. 4,358,887 disclosed the use of sizing rolls positioned upstream of a galvanizing process to create a tube having a round cross-section. Creps further disclosed steel pullout and straightening rolls positioned downstream from the galvanizing process. However, Creps"" straightening rolls are not resilient sizing rolls and put a large force on the round tube, creating deformities. Creps also did not disclose a process for making shaped tubes, that require greater forces placed on them by the sizing rolls, in order to cause the tube to conform to the desired shape.
The U.S. patent to Kajiwara et al., U.S. Pat. No. 5,463,801, disclosed cold rolling mills disposed upstream from a metal plating bath and skin pass mill rolls disposed downstream from the bath. However, the Kajiwara rolls acted to roll the strip of metal prior to galvanization and did not act to size a metal tube. The U.S. patent to Palmer et al., U.S. Pat. No. 4,768,280 disclosed roll forming of metal articles before the galvanizing process and deforming rolls after the galvanizing process, but did not disclose sizing rolls used prior to and after the galvanizing process.
The U.S. patent to Krengel et al., U.S. Pat. No. 3,696,503 disclosed a method for continuously galvanizing strip steel in which a forming roll operation and a reforming roll operation are provided. However, the sizing rolls of Krengel et al. are not resilient rolls and the reforming rolls produce a flat strip and do not size a tube.
One problem associated with the steel sizing rolls of the prior art is that the rolls must exert enough pressure to pull the tube through and that pressure stretches the tube and shrinks the cross-section of the tube. Therefore, the original tube of the prior art must be made somewhat larger than the desired finished product, so that when the stretching and shrinking occurs, the proper cross-section is obtained. The method of the present invention has solved this problem and created several beneficial advantages. For example, the rolls in the first sizing stand of the present invention do most of the sizing work so that the rolls in the second sizing stand need not do as much work and need not pull the galvanized tubes with as much force.
The sizing rolls of the prior art processes generate non-uniform heat in such a way as to shear zinc from the galvanized tube. Also, previous galvanizing processes typically only apply a coating of about 12 microns of zinc in order to prevent cracking of the zinc coating upon sizing by the sizing rolls. An advantage of the present invention is that the second sizing stand comprises resilient sizing rolls, which provide less force on the galvanized tube and prevents cracking of the galvanized surface of the tube. Therefore, the second sizing stand rolls of the present invention perform less work on the galvanized tube which equates to: cooler temperatures; about 30-50% increased mill speeds; thicker galvanized coatings of up to about forty microns; and, fewer cosmetic flaws in the finished galvanized product.
Yet another advantage of the present invention is that prior art processes require the tubes to be uniformly cooled or at the same temperature throughout its length prior to going through the sizing rolls. The sizing rolls of the prior art then work the shaped tubes in a non-uniform manner, creating hot and cold bands along the length of the tube. The non-uniform bands cause straightening problems.
Since the second set of sizing rolls of the present invention grab the conduit with less force, the rolls will not work the hotter areas of the tube (specifically corners) differently than the cooler areas.
Another advantage of the present invention is that zinc from the galvanizing bath does not adhere to the plastic resilient sizing rolls. Therefore, the zinc cannot mark or dent the galvanized tube of the present invention. Also, unlike the sizing rolls of the prior art processes, the second sizing rolls of the present invention are not required to be in a precise progression in order to obtain the correct shape of the final galvanized product.
The present invention discloses a method of sizing a galvanized tube comprising the steps of providing a first sizing station having at least one sizing roll, whereby the roll sizes the tube; galvanizing the tube; providing a second sizing station having at least one resilient sizing roll, whereby the resilient roll sizes the tube.
In another embodiment of the present invention a tube is made according to the process of providing a first sizing station having at least one sizing roll, whereby the roll sizes the tube, galvanizing the tube, and providing a second sizing station having at least one resilient sizing roll, whereby the resilient roll sizes the tube. The tube made by the present invention can be any shape of tubular cross section.
In yet a further embodiment of the present invention a tube sizing production line comprising: a supply station for continuously supplying a metal strip; means for continuously moving the strip along a path; a forming station for forming the metal strip into a tube having a predetermined shape; a welding station for joining the edges of the tube; a first sizing station having at least one set of rolls for sizing the tube; a galvanization station for galvanizing the tube; and a second sizing station having at least one set of resilient rolls for sizing the tube. (A set of rolls requires, at minimum, two opposing rolls, in order to xe2x80x9ccapturexe2x80x9d (confine) the tube. Without opposing rolls, it is not possible to exert sufficient force on the tube to either xe2x80x9cgrabxe2x80x9d or xe2x80x9creshapexe2x80x9d it.)
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following detailed description, appended claims, and accompanying drawings.